In molding of a glass element, the glass element is formed by a series of process steps comprising heating and pressing a die assembly, and cooling the die assembly. Especially when molding an optical glass element having a large curvature or when molding a glass element having a shape hard to release from a die, the molded product can bite into a core or can crack due to thermal contraction of a die assembly upon cooling.
FIGS. 5 and 6 shows a conventional common die structure for use in molding of such a glass element, FIG. 5 illustrating a pair of molding die assemblies in a die-clamped state and FIG. 6 illustrating the pair of molding die assemblies in a die-open state.
In particular, in FIGS. 5 and 6, reference numerals 3 and 5 denote a pair of upper and lower cores each having a molding face for an optical element or a not-shown glass element. Reference numerals 2 and 6 denote a pair of an upper cavity die and a lower cavity die in which the pair of upper and lower cores 3, 5 are slidably fit, respectively. Reference numerals 1 and 7 denote a pair of an upper die plate and a lower die plate which support the pair of upper and lower cores 3, 5 and the pair of upper and lower cavity dies 2, 6. Reference numeral 4 denotes a molded glass element.
A glass material having a yield point of 620° C. may be used as a material for the molded glass element 4. The molded product can be produced by pressing the glass material at 15 kN at 610° C. while closing the die assembly, and then slowly cooling the material to 510° C. Thereafter, the die assembly is opened by 0.5 mm. The molded glass element 4 can remain adhering to the convex surface of the upper core 3 after opening the die assembly, as shown in FIG. 6. This is a problem in the conventional molding die assembly.
An apparatus for molding an optical element has been proposed which can uniformly heat or cool a molding region without damaging the effective molding face of a molding die (Japanese Patent Laid-Open Publication No. 5-286732). In the molding apparatus, at least one hole or grooves is provided in the upper end surface of a middle die, and a spring for upwardly biasing an upper die is provided in the hole or groove. The spring supports the weight of the upper die. An optical element is produced by heating and softening an optical element material between the upper die, the middle die and a lower die, and press-molding the material while applying a pressure to the material by lowering a cylinder on the upper die and thereby pressing on the spring.
The conventional molding die assembly shown in FIGS. 5 and 6 has the drawback that especially when molding an optical glass element having a large curvature or when molding a glass element having a shape hard to release form a die assembly, the molded product can bite into the upper core 3 or the lower core 5 or can crack due to thermal contraction of the die assembly upon cooling, as described above. The cracking of the molded product can damage the upper core 3 or the lower core 5. The molded product can remain adhering to the upper core 3 after opening the die assembly. In addition, there is a case in which the molded product cannot be taken out of the lower core 5. These problems make it difficult to take the molded product out of the die assembly and pose serious obstacles to automated transportation and continuous molding of the molded product.
The patent document, Japanese Patent Laid-Open Publication No. 5-286732, proposes the provision in the middle die of the spring which upwardly biases the upper die so as to balance the load of the upper die, thereby enabling good control of the upper die when it is pressed. The patent document, however, is silent as to how to improve release of a molded product from the die.